Aircraft manufacturers are under continual pressure to reduce the noise produced by aircraft in order to satisfy increasingly stringent noise certification rules. Aircraft engines are a major contributor to overall aircraft noise. Accordingly, aircraft engines in particular have been the target of manufacturers' noise reduction efforts. Aircraft engines have been made significantly quieter as a result of advanced high bypass ratio engines. These engines derive a significant fraction of their total thrust not directly from jet exhaust, but from bypass air which is propelled around the core of the engine by an engine-driven forwardly mounted fan. While this approach has significantly reduced aircraft noise when compared with pure turbojet engines and low bypass ratio engines, engine and aircraft federal regulations nevertheless continue to require further engine noise reductions.
Several techniques have been used to reduce engine exhaust noise. One approach to reducing engine noise is to tailor the amount of mixing between the high velocity gasses exiting the engine, and the surrounding freestream air. A particular technique includes forming “chevrons” at the nozzle exit. Chevrons generally include serrations at the nozzle lip, typically triangular in shape and having some curvature in the lengthwise direction, which slightly immerses them in the adjacent flow. The chevrons can project either inwardly or outwardly, by an amount that is on the order of the upstream boundary layer thickness on the inner or outer surface, respectively. The chevrons can be located at the trailing edge of the nozzle core flow duct (through which the engine core flow is directed), and/or the trailing edge of the fan flow duct, which is arranged annularly around the core flow duct, and through which the fan bypass air passes. The chevrons typically reduce the low-frequency noise by tailoring the rate at which the nozzle flow streams mix with the surrounding freestream air at the length scale of the nozzle diameter. Another technique, which may produce a similar noise reduction is to apply high pressure fluid jets (e.g., microjets) at or near the nozzle exit. While the foregoing approaches have resulted in appreciable noise reduction compared with nozzles that do not include chevrons or fluid injection, further noise reduction is desired to meet community noise standards, and to reduce cabin noise.